The present invention concerns a lighting circuit for HID lamps such as metal halides lamps used for illumination of indoor commercial facilities such as stores and outdoors facilities, light sources for liquid crystal projectors and headlights for use in automobiles or like other vehicles.
HID lamps (High Intensity Discharge Lamps) are also referred to as high luminance discharge lamps or high-pressure discharge lamps and since they are not only excellent in light emission efficiency relative to consumption power but also generate less amount of heat for an identical amount of light and have higher safety compared, for example, with halogen lamps, they have been used, in recent years, in a case where light sources of high luminance are required such as in illumination for indoor commercial facilities and outdoor facilities.
The HID lamp starts discharge by the application of a high voltage at about several kV upon starting and, continues discharge subsequently by applying a relatively low lamp voltage of several tens to several hundreds volts and the HID lamps is put in a lighted state along with increasing lamp voltage.
FIG. 4 shows a general light circuit 41 of lighting an HID lamp by an AC rectangular wave voltage and it comprises a main circuit 2 for applying a lamp voltage at several tens to several hundreds volts to an HID lamp 1, and a starting circuit 3 for applying a high starting voltage at several kilo volts.
The main circuit 2 comprises a rectifier circuit 5 for full wave rectification of a sinusoidal AC wave supplied from an AC power source 4, a power factor improving circuit 6 for converting a rectified pulsative voltage into a smooth DC voltage, a power control circuit comprising a chopper circuit 7A for converting the smooth DC voltage into rectangular pulses of a predetermined pulse width and a smoothing circuit 7B for smoothing the rectangular pulses again into a DC lamp voltage at a predetermined voltage value, and an inverter 9 for converting the obtained DC lamp voltage into an AC rectangular wave voltage at a voltage identical therewith, and the inverter 9 is connected by way of the starting circuit 3 to the HID lamp 1.
The starting circuit 3 has a step-up transformer (not illustrated), which generates a high starting voltage at several kilo volts so as to start discharge between electrodes of the HID lamp 1 when a lighting switch (not illustrated) of the HID lamp 1 is turned on.
In the lighting circuit 41, when the lighting switch (not illustrated) is turned on, a starting voltage at several kilo volts is applied to the HID lamp 1 to start discharge and, subsequent to the start of the discharge, discharge continues by the application of a relatively low lamp voltage of several tens to several hundreds volts supplied from the main circuit 2 and the lamp voltage increases gradually to put the HID lamp into a lighted state.
By the way, although the discharge starting voltage is low while the HID lamp 1 is new, when it is exhausted and contaminants are deposited to the electrodes, it becomes less dischargeable to increase the discharge starting voltage.
Further, when the lamp is lighted again after putting off, since the metal vapor pressure in the inside of the lamp is high, it is in a less dischargeable state and also requires application of high voltage when it is intended to compulsorily start discharge.
As described above, since the discharge starting voltage changes depending on the condition of the lamp, the starting voltage is generally set to about 3-5 kV which is sufficiently higher than the discharge starting voltage so that the lamp can be lit reliably irrespective of the lamp condition.
However, when lamps are lit at a uniformly high starting voltage including HID lamps 1 capable of lighting at a lower starting voltage, this results in a problem of damaging electrodes due to excessively high starting voltage to shorten the product life of the HID lamps 1.
On the other hand, if the lamp 1 per se and wirings therefor have no sufficient insulation property upon starting the HID lamp 1, it may be a worry that large current may flow to injure the main circuit 2 when a high starting voltage is applied. In a case where the number of HID lamps 1 is small, it is possible to check individual HID lamps 1 by periodical maintenance. However, in a case where HID lamps 1 are used for the illumination of a large scale retail store having a large area per one floor such as a department store or a supermarket, since a number of lamps are used, it is impossible to check individual HID lamps 1 one by one before the lamp is disconnected.
In view of the above, the present invention has a technical subject of confirming the insulation property of an HID lamp and a lighting circuit-before application of a fine starting voltage to the HID lamp thereby preventing damages to a main circuit when the starting voltage is applied and starting discharge by the application of a lowest starting voltage in accordance with the characteristics of the HID lamp.
According to the present invention, a lighting circuit for an HID lamp of starting discharge by applying a high starting voltage generated by a step-up transformer of a starting circuit to the HID lamp and then applying a low lamp voltage by way of a main circuit thereby causing continuous discharge, in which a switching element to be turned ON and OFF by a control signal of a predetermined pulse width is connected in series with primary coils in the starting circuit, and a capacitor for absorbing counter-electromotive force generated in the primary coils is connected in parallel with the switching element, and comprises a control section having an insulation property confirming means for confirming that a lamp voltage at a predetermined voltage value is applied by way of the main circuit to the HID lamp and that a current does not flow in the main circuit before application of the starting voltage to the HID lamp, and a starting voltage variable controlling means of increasing the amount of a current flowing in primary coils of the step-up transformer stepwise thereby increasing a secondary voltage generated from the secondary coils stepwise.
According to the present invention, an insulation property of the main circuit for applying the lamp voltage to the HID lamp is confirmed at first before starting discharge by applying a high starting voltage to the HID lamp.
The HID lamp before starting of discharge is in a not-conducted state and, accordingly, even when a voltage is applied to the main circuit, current does not flow unless there is abnormality in the main circuit and the HID lamp.
That is, when a power source is turned on, a lamp voltage at a predetermined voltage value is applied by way of the main circuit to the HID lamp and it can be seen that the main circuit and the HID lamp are normal when the current flowing in the main circuit is zero.
Then, when the voltage and the current are detected to confirm the normality, the starting circuit is started.
When the starting circuit is started, since the amount of the current supplied pulsatively to the primary coils of the step-up transformer increases stepwise, the secondary voltage generated from the secondary coil also pulsatively increases stepwise.
Specifically, a switching element to be turned ON and OFF by a control signal of a predetermined pulse width outputted from the control section is connected in series with primary coils of the step-up transformer, and when the pulse width of the control signal is gradually extended from the predetermined minimum width to maximum width, the amount of current supplied to the primary coils changes and the amount of magnetic field energy accumulated in the core also increases or decreases to change the voltage value generated on the secondary side as well.
Accordingly, since the HID lamp is discharged when the starting voltage increases to a voltage value at which the HID lamp starts discharge, it can be discharged reliably at the minimum starting voltage in accordance with the working time, temperature and other characteristics of the HID lamp.